In a projector or printer or other image display device, in order to raise the resolution of the image, the method of scanning the light from a one-dimensional image display device by a light scanning means and projecting it on an imaging means to form a two-dimensional image has been known (U.S. Pat. No. 5,982,553). As a one-dimensional image display device, a grating light valve (GLV) element proposed by Silicon Light Machines of the U.S. is known (Japanese Patent No. 3164824, U.S. Pat. No. 5,841,579).
A GLV element is a micro-machine phase reflection type grating utilizing the diffraction of light. It reflects incident light as reflected light having no optical path difference in a first state and reflects the incident light giving it an optical path difference in a second state to generate diffracted light by the diffraction phenomenon.
If applying GLV elements to an image display device, the elements exhibit a dark state when the diffracted light is not generated and give diffracted light becoming a digital image when the incident light is diffracted. Namely, by scanning the diffracted light from one-dimensionally arrayed GLV elements by a scan mirror, a two-dimensional image is obtained.
Compared with a usual two-dimensional image display device using a liquid crystal panel etc., in the case of an image display device using GLV elements, the number of elements in the vertical direction becomes the same. In the horizontal direction, however, single one-dimensionally arrayed GLV elements are sufficient, so the structure of the element required for displaying a two-dimensional image becomes simpler. Further, the elongated ribbon-like electrode portions of GLV elements referred to as “ribbon reflective members” are very small in size (for example, about 1×40 μm), have a high switching speed of reflection and diffraction and a wide bandwidth, and, when applying GLV elements to an image display device, enable display of a high resolution and a high speed. Further, GLV elements operate with a low applied voltage, so consume little electric power. If considering the above, if using GLV elements, realization of an image display device having a very small size, a high resolution, and a high response speed can be expected.
The basic configuration and operation principle of a GLV element will be explained in brief by referring to FIG. 1 to FIG. 3.
FIG. 1 is a partial perspective view of a GLV element for displaying a one-dimensional image.
As shown in FIG. 1, a GLV element 1 has a substrate 12 serving as a common electrode and ribbon reflective members 10a, 11a, 10b, 11b, 10c, 11c, and 10d comprised of elongated ribbon-like (stripe-like) reflective members arranged facing each other across predetermined intervals (gaps). The ribbon reflective members 10a, 11a, 10b, 11b, 10c, 11c, and 10d have reflective coatings (not illustrated) for reflecting light formed on their upper surfaces and become the reflective members for reflecting light incident from above.
The ribbon reflective members 10a, 11a, 10b, 11b, 10c, 11c, and 10d are grouped into alternately arranged members. The ribbon reflective members 11a, 11b, and 11c (indicated together as “11”) of a first group and the common electrode substrate 12 are held at a ground potential. Since they have the same potential, no electrostatic force is induced with the common electrode substrate 12 and they do not move (are not displaced) toward the common electrode substrate 12. The ribbon reflective members 11 which do not move and are fixed in this way will be referred to as “fixed ribbon reflective members”. The ribbon reflective members 10a, 10b, 10c, and 10d (indicated together as “10”) of a second group become the ground potential in the first state. They are located at the same reflection plane as the fixed ribbon reflective members 11 of the first group, but when a drive voltage PWR is applied in a second state, electrostatic force acts between the common electrode substrate 12 and the ribbon reflective members 10a, 10b, 10c, and 10d to which the drive voltage PWR is applied and therefore the ribbon reflective members 10 displace and approach the common electrode substrate 12. When application of the drive voltage PWR to the ribbon reflective members 10 is stopped, the ribbon reflective members 10 return to their original horizontal locations. Accordingly, the ribbon reflective members 10 will be referred to as “moveable ribbon reflective members”.
The ribbon reflective members and the common electrode substrate 12 have conductivity. Particularly, the moveable ribbon reflective members have flexibility for the displacement as described above. The common electrode substrate 12 is fixed. Of course, the ribbon reflective members have reflection characteristics for reflecting the incident light.
An example of typical dimensions of a ribbon reflective member will be explained next. For example, a width of each ribbon reflective member is 3 to 4 μm, a gap between adjacent ribbon reflective members is about 0.6 μm, and a length of the ribbon reflective member is about 200 to 400 μm.
A GLV element configured by a common electrode substrate 12 and a plurality of ribbon reflective members can be used to form one pixel by one such set of parts. For example, one pixel can be expressed by the six adjacent ribbon reflective members 10a, 11a, 10b, 11b, 10c, and 11c shown in FIG. 1. In this case, one pixel=s worth of width is 21 (3×6+0.6×5) μm to 27 (4×6+0.6×5) μm. When taking the average, one pixel=s worth of width is about 24 μm.
For example, in the currently being commercialized GLV elements for displaying 1080 pixels, a large 1080 pixels=worth of ribbon reflective members are arranged along the horizontal direction of FIG. 1. Such a GLV element array can be fabricated by microsemiconductor production technology.
The method of operation of a GLV element will be explained next.
(1) GLV Off State
If the common electrode substrate 12 is made the ground potential and the fixed ribbon reflective members 11 are also made the ground potential and the application of the drive voltage PWR to the moveable ribbon reflective members 10 is stopped to make them the ground potential, the planes of the moveable ribbon reflective members 10 and the fixed ribbon reflective members 11 become the same.
FIG. 2 is a sectional view of the horizontal direction of the GLV element 1 when making the moveable ribbon reflective members 10 the ground potential. This state will be referred to as the “off state” of the GLV element (inactive state or first state). In other words, in the off state of the GLV element, the moveable ribbon reflective members 10 and the fixed ribbon reflective members 11 are substantially located in the same plane across the above-mentioned gaps between planar ribbon reflective members. Namely, all ribbon reflective members maintain a certain distance from the substrate 12 and form almost the same reflection plane.
When the illumination light (incident light Li) is incident on the GLV element from above the ribbon reflective members in the off state illustrated in FIG. 2, it is reflected at the ribbon reflective members 10a, 11a, 10b, 11b, 10c, 11c, and 10d forming almost the same reflection plane and no optical path difference occurs in the reflected lights Lr. That is, all ribbon reflective members together function as a plane mirror which reflects incident illumination light (incident light Li) with almost no diffraction and polarization. The off state of a GLV element corresponds to a dark state of the screen when applying the GLV element to an image display device. The display surface becomes black.
(2) GLV on State
If making the common electrode substrate 12 the ground potential and also making the fixed ribbon reflective members 11 the ground potential and applying the drive voltage PWR to the moveable ribbon reflective members 10, electrostatic force acts between the common electrode substrate 12 and the moveable ribbon reflective members 10, the moveable ribbon reflective members 10 approach the common electrode substrate 12 (move downward), and therefore the moveable ribbon reflective members 10 approach the common electrode substrate 12 from the plane of the fixed ribbon reflective members 11. The moveable ribbon reflective members 10 have conductivity and flexibility enabling them to displace when the drive voltage PWR is applied in this way and to return when the application of the drive voltage PWR is stopped (GLV off state).
FIG. 3 is a sectional view of the horizontal direction of the GLV element 1 when applying a drive voltage PWR to the moveable ribbon reflective members 10. This state will be referred to as the “on state” of the GLV (active state or second state).
As shown in FIG. 3, the moveable ribbon reflective members 10 to which the drive voltage PWR is applied are pulled down to the substrate 12 by the electrostatic force and are separated from the plane of the fixed ribbon reflective members 11 by exactly a predetermined distance suitable for causing a diffraction phenomenon, for example, exactly λ/4. λ is the wavelength of the incident light Li. As an example, when λ=532 nm, the amount of movement of the moveable ribbon reflective members becomes λ/4=133 nm.
When the illumination light Li is incident on the GLV element in the on state illustrated in FIG. 3, the total optical path difference between the light reflected at the moveable ribbon reflective members 10 and the light reflected at the fixed ribbon reflective members 11 becomes the half wavelength (λ/2) and a diffraction phenomenon occurs. Namely, the adjoining moveable ribbon reflective members and fixed ribbon reflective members form a cyclic structure diffracting the incident light Li. In the second state, the GLV element functions as a reflection type grating.
The reflected lights (0 order lights L0) interfere with and cancel out each other, while the ±1 order diffracted lights L--1 and L-+1, ±2 order diffracted lights L--2 and L-+2, and other order diffracted lights are produced.
For example, the ±1 order diffracted lights L--1 and L-+1 pass through a not illustrated optical system in the image display device and are focused on the screen or other display surface of the image display device. The optical system of the image display device is configured so that the 0 order light L0 is blocked by for example a space filter etc. and does not reach the display surface of the image display device.
When parallel light L0 forming an incident angle θi with respect to the plane of the ribbon reflective members is incident on the GLV element in the on state, a diffraction angle θm of a generated m order diffracted light can be represented by the following equation 1:sin(θm)=sin(θi)+(m·λ/D)  (1)                where, D is a predetermined distance (lattice pitch) between the ribbon reflective members forming the same group of the GLV element shown in FIG. 3.        
When the incident light Li is vertically incident upon the surface of the GLV element (θi=0), the order of the ±1 order diffracted light having the highest intensity becomes m=1, so the diffraction angle θ1 becomes as in the following equation 2:sin(θ1)=λ/D  (2)
When using a GLV element for an image display device, for example, at the time of display of black, the GLV element is used in the off state, while at the time of display of a color other than black, the GLV element is used in the on state. The display of the various colors when the GLV element is in the on state is determined by the light incident upon the GLV element.
In an image display device using a GLV element array functioning as such a one-dimensional image display device, in comparison with the usual two-dimensional image display device, for example, a projection type display device using a liquid crystal panel or the like, since there are no borders between pixels in the GLV element array itself, an extremely smooth and natural image can be expressed. Further, by using three primary color, that is, red, green, and blue lasers as the light sources and mixing these lights, it is possible to express images of an extremely broad and natural color reproduction range and obtain other superior aspects of display performance never before seen in the past. An image display device using a GLV element array is expected to realize a high contrast of for example 1000:1 or more.
For example, for a 1080×1920 pixel image display device, it is not easy in practice to realize a good image display for all of the 1080×1920 pixels obtained by making light strike the 1080 pixels=worth of GLV elements to generate the above diffracted light and scanning the diffracted light inside the image display device by a scanner. The reason is that due to imperfections in the production process etc., there is nonuniformity in the formation of the nanometer size ribbon reflective members and therefore there is some variation (fluctuation) in the ribbon reflective members of the 1080 pixels included in one GLV element array. As such variation, there are for example variation of the optical path length due to variation of the amount of movement of the plurality of moveable ribbon reflective members in the on state of a GLV element due to error (variation) of the drive voltage PWR for moving the ribbon reflective members at the nanometer level, occurrence of nonuniform planes of the ribbon reflective members in the off state due to variation of heights of the ribbon reflective members in the off state of the GLV element caused by the production technique and the other distortion, occurrence of scattering of light due to diffraction of the light outside of the effective illumination area, unevenness on the reflection surfaces of the ribbon reflective members and various deposits on the ribbon reflective members, and the influence of nonuniform secondary illumination light. These become factors deteriorating the image quality. These factors cause fluctuation particularly in brightness in the dark state (off state of GLV element).
Particularly, since the light is made incident upon the GLV element array comprised of the one-dimensional image display device to generate the diffracted light, and the diffracted light is scanned inside the image display device by a scanner to obtain a two-dimensional image, if there is variation in the image qualities of the pixels, pixels having a bad contrast will leave stripe like noise on the display surface by the scanning, so an image display device having a low image quality results.
Note that a GLV element functioning as a reflective member for reflecting all of the incident light in the first state and functioning as a grating for diffracting the incident light in the second state mentioned above will be referred to as a “light reflection and diffraction element” below.